1. Field of the Invention
The present invention relates to a compression molding method for a cutting insert, and more particularly, to a compression molding method in which the accuracy of contours (diameter of inscribed circle) of the upper and lower surfaces of a cutting insert is improved.
2. Description of the Related Art
In a conventional compression molding method for molding powder, a certain volume of molding powder is filled into a molding space defined by a die and a pair of punches, upper and lower, and compression molding is performed by means of the upper and lower punches. In this known compression molding method, priority is given to the point that each punch is stopped at a predetermined position.
Further, there are powder molding machines in which a molding section is composed of a die and punches. In these powder molding machines, each punch is mechanically driven by a ball screw, and the drive mechanism is connected with a servomotor and provided with a sensor for detecting the compressive force of the punch. Some powder molding machines are provided with control means that compares a measured value obtained by the sensor and a predetermined reference value and controls the servomotor so that the measured value corresponds to the reference value. These powder molding machines have an effect that a compact can be compression-molded to a uniform density (Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 1-181997).
According to the compression molding method in which the upper and lower punches are stopped at the predetermined positions, a fixed filling weight is obtained by making the volume of molding powder constant. The shape, operation setting, etc., of a filling device are optimized in order to make the volume of molding powder constant. If the particle size of the molding powder is subject to variation, however, a problem is caused that the density of the compact becomes so uneven that the dimensional accuracy after sintering is reduced. Thus, if a cutting insert formed of cemented carbide, cermet, etc., is used as a cutting edge of a cutting tool, therefore, the edge dimensions of the cutting edge considerably vary at the time of replacement, so that the machining accuracy is reduced. Further, the shape, operation setting, etc., of the filling device must be separately managed for each of different molding powder particle sizes, which is troublesome.
In the compression molding method using the powder molding machine described in Jpn. Pat. Appln. KOKAI Publication No. 1-181997 (see FIG. 1), the control is performed based on the compressive force between the die and punches, so that the distance between the upper and lower punches fluctuates depending on fluctuations of the fill of the molding powder. In order to suppress this fluctuation, the molding powder volume must be accurately controlled. If a compression operation is performed without filling the molding powder due to malfunctioning of the device, moreover, the upper and lower punches may collide with each other and break the die. If the die is not broken, the dimensions after sintering may be out of tolerance, thereby causing defective production.